Connecting to a PLC not made by Allen-Bradley and Modicon

I see Allen-Bradley and Modicon protocols listed, can I connect to a PLC not made by those manufacturers?

Yes you can. Most PLC manufacturers have built provisions for connecting to other devices by the Modbus protocol. Raco has provided some information for GE, Bristol Babcock & Siemens Products. The best source, though, is from the manufacturer of the product you are considering.